1. Field of the Invention
The present invention relates to boiling water nuclear reactors. More particularly, the present invention relates to a safety system for controlling reactivity by injecting liquid boron into a reactor core.
2. Description of the Prior Art
All nuclear power plants, both boiling water reactors (BWRs) and pressurized water reactors (PWRs) are designed for a forty year life time. In that life time, the plants are designed to handle a full series of anticipated transients--a component failure, an operator error, and anything else that would upset the power plant from its normal steady state operation. When a transient occurs, the power plant is designed to shut down by a process called scramming, i.e. rapid insertion of control rods into the plant reactor core.
For more than a decade, the members of Nuclear Regulatory Commission (NRC) have been concerned that an anticipated transient may occur where the control rods do not respond to a scram demand. That is, the reactor is in an upset condition and cannot be shut down in the normal mode by the insertion of control rods into the core. This concern by the NRC has resulted in the promulgation of "draft rules" imposing additional safety requirements upon nuclear power plant operators. The additional safety requirements serve to resolve the NRC's concern over anticipated transients without scram (ATWS).
Normal shut down (scram) inserts solid rods containing a reactivity control agent, such as boron, into the reactor core. The alternate means for controlling a reactor comprises a holding tank, outside of the reactor vessel, containing a solution having a liquid form of boron, such as sodium pentaborate. In the event of an ATWs event, personnel at the power plant normally operate a switch to activate a pump. The pump transfers the sodium pentaborate from the holding tank into the reactor core, thereby shutting down the reactor.
Inserting rods into a reactor normally shuts down the reactor, and the power can be restored by removing the rods within a very short time. This is because the rods are designed to be inserted and removed from the reactor. Pumping liquid sodium pentaborate into the reactor requires a major reactor cleanup effort that could take up to six months to complete. Since the average cost of nuclear plant down time is $400,000.00 per day (1983), reactor personnel generally think twice before actuating the sodium pentaborate pumps.
To arrive at the decision to initiate pumping of sodium pentaborate into a reactor core, plant personnel generally refer to three parameters--dome pressure, water level, and average power. Certain procedures are available to plant personnel that may prevent an upset of the reactor in the event that an ATWS event occurs. These procedures involve cutting the recirculation of water to the system by tripping the recirculation pumps, limiting the feed water flow, and alternate rod insertion. If all of these procedures fail, the plant personnel have the option to manually operate the standby liquid control system (SLCS) to pump liquid sodium pentaborate into the reactor.
The NRC rules require that such parameters as peak fuel temperature, peak suppression pool temperature, and radiation release to the public not exceed preset limits. The protocol, as of the origin of this invention, has not been dictated. Accordingly, the protocol herein was developed by the inventors herein, and not mandated by regulation.